system.scm: add "SetEnv" to sshd•••Explicitly set `GIT_CONFIG_GLOBAL=/etc/gitconfig` so that one can push
with options (e.g. `git push -o ci.skip`) without having to authorize
Git repositories individually, as `/etc/gitconfig` already contains
that.
{git-post-receive,cronjob}.sh: Improve portability•••- don't use GNU's date(1) "%s" proprietary option, and leverage AWK's
default behaviour of `srand()` to get the seconds since UNIX epoch;
- don't rely on GNU's sed(1) "-u" proprietary option of unbuffering, and
do a manual `while` loop instead.